lxd/instance: Don't assume root disk is called "root" when copying snapshots from a source instance
lxd/instance: Don't assume root disk is called "root" when copying snapshots from a source instance

Try to avoid modifying the new snapshot's disk config, but where needed, modify the pool property to match target pool.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,atxwebs/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,atxwebs/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,atxwebs/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,atxwebs/lxd,lxc/lxd